I think the hangup there is, someone has to be convicted of a crime before they can be pardoned. And a conviction requires an investigation.
You do not have to be convicted of a crime to be pardoned. You do not even have to be accused of a crime to be pardoned. http://en.wikipedia.org/wiki/Pardon#United_States (I'm assuming that wikipedia is correct, but strong evidence is that Nixon was never charged with anything, but was pardoned by Ford.)